(1) Field of the Invention
The present invention relates to an arbitrary and limitless polarization controller, a polarization-mode dispersion compensator using the same, and an arbitrary and limitless polarization controlling method, which are used in optical communication and the like.
(2) Description of Related Art
A polarization controller can freely change a polarization state of incident light from an arbitrary state to another arbitrary state. As it is well known, the polarization state of light can be expressed by Stokes parameters S0 (polarization intensity), S1 (horizontal linear polarization (TE) component), S2 (oblique 45° direction linear polarization component), and S3 (clockwise circular polarization component). At complete polarization, that is, at a DOP (degree of polarization)=1, the following equation is established.S02=S12+S22+S32 
Therefore, each of the Stokes parameters S1, S2, and S3 expressing the polarization states are positioned at a point on a sphere whose radius is the intensity S0. If only the polarization state is focused on, by considering light of the unit intensity (S0=1), the polarization state can be expressed by one point on a unit sphere in which the S1, S2, and S3 are respective three axes in an orthogonal coordinate system (refer to FIG. 18). This is “Poincare sphere”.
If the Poincare sphere shown in FIG. 18 were regarded to be a terrestrial globe, the equator 110 shows linear polarization, the geographical north pole NP shows clockwise circular polarization, and the geographical south pole SP shows counter-clockwise circular polarization. In addition, the point A shows horizontal linear polarization (TE mode), the point B shows vertical linear polarization (TM mode), and the point C shows oblique 45° direction linear polarization. Further, polarization anywhere besides on the equator 110, the geographical north pole NP, and the geographical south pole SP shows ellipse polarization.
That is, in order for the polarization controller to change the polarization state of incident light freely from an arbitrary state to another arbitrary state, it is necessary that an arbitrary point can be changed (converted) freely to another arbitrary point on the Poincare sphere.
In one method, an electro-optical effect was used to realize a polarization controller. The method used a variable polarization light element (hereinafter, in some cases, simply referred to as an element). In the variable polarization light element, wave plate elements arrayed in plural stages could freely rotate the azimuth (angle for the vibration direction of the incident light) of the optical axis while the phase shift amount was kept constant. Especially, it is known that a polarization controller having the arbitrary and limitless property of a three stage structure can be realized by using three elements: a quarter-wave plate, a half-wave plate, and a quarter-wave plate, whose optical axes can be freely rotated.
In Japanese Patent Laid-Open (Kokai) SHO 63-118709, a polarization controller provides (1) a first ellipticity conversion element that can change (rotate) a point on the Poincare sphere around the S1 axis or the S2 axis by the angle of ellipticity control amount δφ1 corresponding to a control voltage V1 (or a control current I1). The polarization controller of SHO 63-118709 also provides (2) a phase shifter element that can change (rotate) a point on the Poincare sphere corresponding to the polarization state of light from the first ellipticity conversion element around the S2 axis by the angle of phase control amount δφ2 corresponding to a control voltage V2 (or a control current I2). Finally, the polarization controller of SHO 63-118709 provides (3) a second ellipticity conversion element that can change (rotate) a point on the Poincare sphere corresponding to the polarization state of light from the phase shifter element around the S1 axis or the S3 axis by the angle of ellipticity control amount δφ3 corresponding to a control voltage V3 (or a control current I3).
According to the polarization controller of SHO 63-118709, although the above-mentioned ellipticity control amount δφ1 is limited to a range “0 to π”, the phase control amount δφ2 is limited to a range “−π to π”, and the ellipticity control amount δφ3 is limited to a range “−π to 0”, a polarization state expressed by an arbitrary point on the Poincare sphere can be continuously changed (converted) to another polarization state expressed by another arbitrary point. In particular, a polarization state expressed by an arbitrary point on the Poincare sphere can be continuously changed to another polarization state by properly controlling the δφ1, δφ2, and δφ3, that is, by properly controlling the control voltages V1, V2, and V3 (or the control currents I1, I2, and I3).
In addition, as another aspect of the polarization controller of SHO 63-118709, if an element, which can freely change the azimuth {circle around (-)} in “0 to π/2” by rotating the optical axis and at the same time can freely change the phase shift amount in “0 to λ” (λ shows a wavelength of input light), were used, a polarization controller of one stage structure could be realized. This principle can be considered, for example, in a Poincare sphere expression shown in FIG. 19. That is, in the Poincare sphere expression, if the phase shift amount (phase shift between TE component and TM component) were changed by making the element function as a phase element whose azimuth of optical axis is 0°, the point S on the Poincare sphere 100 could be changed (rotated) to the point S′ corresponding to the phase shift amount (refer to the alternate long and short dash line arrow 101). Moreover, by using the S1 axis as the rotation axis, when the phase shift amount becomes λ, the point S can be exactly traveled around (returned to the original polarization state). In this way, if the phase shift amount were changed by making the element function as a phase element whose azimuth of optical axis is 45°, a point on the Poincare sphere 100 can be changed (rotated) corresponding to the phase shift amount by using the S2 axis as the rotation axis.
On the other hand, if the azimuth were changed by rotating the optical axis of the element, the above-mentioned rotation axis (S1 axis or S2 axis) on the Poincare sphere 100 would rotate along the equator [that is, by making the earth axis (S3 axis) of the Poincare sphere its center] (refer to the alternate long and short dash line arrow 102). Therefore, the azimuth of the optical axis of the element would change so that a circle 103 drawn so that the rotation axis of the Poincare sphere is made a center passes through the two points of the input polarization state and the output polarization state. Moreover, if a phase shift amount which only changes between the two points were given, an arbitrary polarization state could be converted to another arbitrary polarization state.
In order for a polarization controller to have an arbitrary and limitless property, as mentioned above, the ranges in which the rotation of the optical axis of the element and the phase shift amount of the element are changed must not have limitations. Since, however, the phase shift amount is generally changed by applying a voltage (or a current), there is a limitation. In order to solve this, it is necessary for the change of the phase shift amount to be switched in an inverse direction by rotating the rotation axis on the Poincare sphere by π. If this switching were executed at the time when the phase shift amount became λ, the rotation axis on the Poincare sphere could be rotated without changing the polarization state, consequently, the arbitrary and limitless control could be realized.
As mentioned above, at the polarization controller of a one state structure, the number of stages is small, therefore, the number of electrodes to be controlled can be reduced. However, if arbitrary and limitless control were to be realized in such a polarization controller, using the above-mentioned technology, the phase shift amount is required to be λ in order to switch the rotation axis on the Poincare sphere in any case. Consequently, in order to further reduce the power consumption, it is necessary to make the phase shift amount small.
An optical polarization control element which can be driven by a low voltage is disclosed by Japanese Patent No. 2646558. In the optical polarization control element described in Japanese Patent No. 2646558, a thin film is formed on the surface of an electro-optical crystal substrate in which an optical waveguide is formed. The thin film is of lower dielectric constant than that of the substrate. Electrodes are formed on the thin film and on the back of the substrate, making low voltage driving possible. In addition, Japanese Patent Laid-Open (Kokai) 2001-324627, describes a polarization controller which is applied to an optical receiver and wavelength dispersion of received light signals is compensated.
The present invention is created by considering the above-mentioned problem, and an object of the present invention is to realize arbitrary and limitless control of polarization by a phase shift amount of less than the wavelength λ of input light.